1. Field of the Invention
The invention relates to a lead for an implanted medical device in which the lead includes a wire for placing an electrical signal source in electrical communication with an electrode at a distal tip of the lead. The lead is substantially transparent to radio frequency waves in clinically-applicable magnetic resonance environments to avoid radio frequency heating effects.
2. Description of the Related Art
Magnetic resonance imaging (MRI) for patients with 1neurostimulators has advantages and limitations. Implanted medical devices such as cardioverter-defibrillators, pacemakers, spinal cord stimulators, and deep brain stimulation (DBS) have become well-accepted therapeutic options to treat a wide range of medical conditions and contribute to improved quality of life. [Ref. 1] Many patients with implanted devices can benefit from MRI, which is the diagnostic tool of choice for monitoring structural changes in the body, as well as diagnosing many common illnesses including cancer, cardiovascular disease, and trauma. Additionally, functional MRI is becoming more prevalent in assessing brain function and cognitive disorders [Ref. 2,3]. However, approximately 300,000 patients with implanted or partially implanted medical devices are denied MRI each year because of safety concerns [Ref. 4]. A major concern when performing MRI examinations in patients with electrically conductive implants is the increase in induced currents (“antenna effect”) along conductive leads in the body that are exposed to the radiofrequency (RF) waves of the MRI. The increase in current flow into the tissue at the point of contact with the lead (i.e., the electrodes) causes a large amount of RF energy to be absorbed in the tissue, which in turn causes surges in temperatures that can lead to injury [Ref. 5-12]. Temperature increases of up to 25° C. were measured near a DBS 3389 lead (Medtronic, Inc., Minneapolis, Minn.) in an in-vitro gel phantom at 1.5 T MRI [Ref. 13]. Additionally, increases of up to 30° C. were measured with the Medtronic 3389 lead in a swine head at 9.4 T [Ref. 14]. More importantly, two cases of serious, permanent neurological injury, after MRI exposure at 1.0 T in patients with DBS implants, have been reported [Ref. 15,16]. In both cases the manufacturer guidelines were not followed and in one case the patient developed paralysis following MRI examination [Ref. 16]. The lack of access to MRI is expensive to society because patients are denied the benefits of screening and accurate diagnosis.
A class of implantable devices—defined as “MR Conditional” [Ref. 17]—have been shown to pose no known hazards in the MRI environment when operated with specified conditions. For example, the Activa® DBS system (Medtronic, Inc., Minneapolis, Minn.) is approved for use in MRI [Ref. 18] with several conditions [Ref. 19], including limited static and gradient magnetic fields, use of low power sequences, and specific RF coils. These conditions, however, are restrictive. The limit for power absorbed by the patient's head is over 30-fold less than typical values allowed, which restricts the number, the type, and quality of MRI scans that can be performed in a given session. The most commonly used transmit body coils are not allowed, excluding the possibility of using MRI to diagnose morbidities in the human torso (e.g., breast cancer, back pain). Additionally, the conditions exclude the use of 3.0 T MRI systems, which are predominately used in clinical [Ref. 20] and research applications [Ref. 21,22].
Therefore, a need exists for an improved a lead for an implanted medical device wherein the lead is substantially transparent to radio frequency waves in clinically-applicable magnetic resonance environments to avoid radio frequency heating effects.